The importance of macular pigment to the health of the eye has prompted development and interest in methods for measuring its density or concentration in the retina. Prior systems and methods, however, have either been based upon equipment which is not commonly available, is time consuming, or complicated and expensive.
With reference now to FIG. 1, a diagrammatic view of an eye, generally referred to by the reference number 10, is shown. The eye 10 includes a cornea 12 which is a transparent front part of the eye that covers the iris and pupil 14 which is the variable-size black circular or slit-shaped opening in the center of the iris that regulates the amount of light that enters the eye. The lens 16 is a transparent biconvex structure in the eye that, along with the cornea 12, helps to refract light to be focused on the retina 18. The retina is a thin layer of neural cells that line the back of the eyeball which captures light and transforms it into electrical signals for the brain. It has many blood vessels 20 to nourish it. The fovea and macular region, referred to by the reference number 22, is a portion of the eye used for color vision and fine detail vision. The retinal pigmented epithelium (RPE) 24 is the pigmented cell layer just outside the neurosensory retina 18 that nourishes the retinal visual cells. It is firmly attached to an underlying choroid 26 which is a vascular layer of the eye 10 lined between the retina 18 and the sclera. The choroid 26 provides oxygen and nourishment to the outer layers of the retina 18.
Many diseases of the eye are related to the retina and there have been developed methodologies to treat such diseases and conditions. Some forms of phototherapy, such as photostimulation and photocoagulation, rely upon heating of the retinal tissue to create their therapeutic effects. Excessive heating can damage or even destroy retinal tissue, which in some treatment methodologies is intentional but in others is avoided. It has been found that abnormal levels of pigmentation, particularly levels or concentrations of melanin within the RPE, can cause unanticipated and excessive heat during such treatments and potentially damage the retinal tissue.
Melanin in the eye has many important functions which are not yet completely understood. Melanin in the eye provides protection to the eye by absorbing harmful ultraviolet radiation. Melanin promotes visual acuity by scattering stray light away from the rods and cones and absorbing light reflected from the back of the eye. Melanin also serves as an antioxidant to aid in the prevention of retinal diseases, such as age-related macular degeneration.
Many of these properties result from the fact that the absorption spectrum of melanin is very broad. In this respect, it is unique among pigments. Many mechanisms have been suggested for this unique behavior. As examples, the broadband absorption has been attributed to chemical heterogeneity, amorphous semiconducting, and scattering. However, it has been shown that scattering losses only account for a few percent of the broadband attenuation. There are also problems with the chemical heterogeneity and amorphous semiconducting hypothesis. Some have proposed polymeric charge hopping. Others have pointed out the importance of hydration and introducing free radicals into melanin. Yet others have suggested that melanin excitons may play a role in its broadband absorption. There does not appear to be universal agreement that any particular explanation can account for all of melanin's electrical and optical properties.
As indicated above, melanin within the eye serves many important functions. The determination of the levels or concentrations of melanin within the eye can be important to ascertain. For example, phototherapy laser treatments of eye diseases may be based on inducing temperature rises in the RPE, which activates the eye's natural repair mechanisms. In the near infrared, this results from the absorption of the infrared radiation by the melanin pigment in the RPE. Considerable melanin also exists in the choroid behind the RPE, but absorption by the choroidal melanin does not play a significant role in raising the temperature of the RPE due to the lack of diffusive heat transfer to the RPE during the relatively short treatment times and due to the convective cooling by the blood vessels in the choroid and the choriocapillaris.
In laser true subthreshold damage phototherapy treatments of eye diseases, which avoid retinal damage, the laser treatment is effective as long as the temperature rise does not exceed the order of 10° C. This temperature rise limitation determines the maximum laser energy that can be absorbed by the RPE during the treatment time. A possible concern, however, is that for laser powers that are suitable for most patients, the temperature rise can exceed the threshold for damage if the patient's RPE melanin content or concentration is abnormally too large.
Accordingly, there is a continuing need for a simple and relatively inexpensive process for determining melanin levels or concentrations within the eye, and particularly within the RPE of the eye, so that one or more treatment parameters of the retinal phototherapy treatment can be adjusted as needed to avoid damaging patient's eyes who have an abnormally large content or concentration of melanin in the RPE. The present invention fulfills these needs, and provides other related advantages